Use the `trace` decorator everywhere

Change the `trace` decorator to log at DEBUG level instead of
monkey patching the logger to introduce a new LOG_LEVEL.

Also, remove redundant logging.
With 10MB objects, without or with the call to logger.debug(), I get :+1: 
200 PUTS **FINAL** [0 failures], 8.8/s

I'll remove the line anyway because it doesn't help much.